1. Field of the Invention
The present invention relates to a method and an apparatus for accurately measuring or detecting the roughness or minute undulation of a surface of a subject, by utilizing optical heterodyne interference, even where the subject surface has a relatively large variation in the height or depth of depressions and projections which define the undulation.
2. Discussion of the Prior Art
There is known an optical apparatus or instrument for measuring, detecting or inspecting the roughness, cross sectional profile or minute undulation of a surface of a subject, by utilizing a heterodyne interference between a reference beat beam having a fixed frequency, and a measuring beat beam whose frequency varies due to changes in the height or depth of the depressions and projections on the subject surface. Namely, a light beam is converged on the subject surface, by an object lens, and is reflected by the subject surface. The reflected light is used as the measuring beat beam whose frequency varies due to a variation in its phase which arises from the height or depth variation of the subject surface. This type of measuring apparatus suffers from a low measuring resolution in the direction parallel to the subject surface, where the measuring spots on the subject surface are not precisely positioned at the focal point of the object lens. Further, the deviation of the subject surface from the focal point of the object lens may cause an measuring error (measured height of projections or depth of depressions being smaller or larger than the actual height or depth), where the height or depth of the projections or depressions is larger than the wavelength of the measuring light beam incident upon the subject surface.